Built big, tough box (8)
I believe the answer is:
heavyset
'built' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?
'big tough box' is the wordplay.
'big' becomes 'heavy' (similar in meaning).
'tough box' becomes 'set' (I can't justify this - if you can you should believe this answer much more).
'heavy'+'set'='HEAVYSET'
Can you help me to learn more?
(Other definitions for heavyset that I've seen before include "Mesomorphic" , "Stocky in stature" .)